1. Field of the Invention
The present invention relates to a gradient magnetic field coil for defining a tomographic plane in a magnetic resonance imaging (MRI) apparatus which is used for medical diagnosis by means of generating a computed tomography using nuclear magnetic resonance phenomena.
2. Description of the Prior Art
FIG. 1 is a perspective schematic view representing a principle of a conventional gradient magnetic coil where four saddle-type encircling coils are placed in symmetrical positions. In FIG. 1, a gradient magnetic coil 10 is composed of four saddle-type encircling coils 11, 12, 13, and 14. In the case of defining the origin O to be the center of a uniform static magnetic field and defining the x axis, the y axis and the z axis to be shown in FIG. 1, the saddle-type encircling coil 11 is composed of circular arc parts 111 and 113 which are placed along the cylindrical surface formed to be symmetrical with respect to the z axis and two straight parts 112 connected to circular arc parts 111 and 113, and an electric current flows in the direction depicted by arrows in the figure. The saddle-type encircling coil 12 is composed of two circular arc parts 121 and 123 and two straight parts 122 connected to these circular arc parts 121 and 123, and its configuration, layout and current flow direction are symmetrical to those of the saddle-type encircling coil 11 with respect to the z-x plane which is defined by the z axis and the x axis. Saddle-type encircling coils 13 and 14 are symmetrical to saddle-type encircling coils 11 and 12 with respect to x-y plane which is defined by the x axis and the y axis, respectively. As for electric current direction, for example, electric current directions in two circular arc parts 111 and 131 are identical to each other and electric current directions in circular arc parts 121 and 141 are identical to each other. Each electric current in circular arc parts 111, 131, 121 and 141 directs from the front side to the back side in FIG. 1, and circular arc parts 111, 131, 121 and 141 mainly establish a gradient magnetic field by superpositioning magnetic fields generated by them onto a uniform magnetic field in the neighboring area of the origin O. The gradient magnetic field coil 10 of FIG. 1 is a y-direction gradient magnetic field coil which generates a gradient magnetic field the z-direction component of which is proportional to a displacement in the y axis direction. Since a gradient magnetic field to be formed with a gradient magnetic field coil as shown in FIG. 1, which comprises only a set of saddle-type encircling coils 11, 12, 13 and 14, does not have good linearity characteristics of magnetic field to be required for practical MRI apparatus, it is commonly practiced that an actual gradient magnetic coil is composed of a plurality of sets of saddle-type coils.
FIG. 2 is a perspective schematic view of a gradient magnetic field coil composed of three sets of saddle-type encircling coils (Japanese Patent application Laying-open No. 229808/1987). In FIG. 2, a gradient magnetic field coil is composed of three sets 1, 2, and 3 of saddle-type encircling coils, each set being formed in a determined size and shape and placed at a determined position optimized by numerical calculations in order to generate a magnetic field having good linearity characteristics. Defining l1 to be a distance between the x-y plane and the circular arc part 111 of a saddle-type encircling coil 1 which part is placed adjacent to the x-y plane, defining l2 to be a distance between the x-y plane and the circular arc part 211 of a saddle-type encircling coil 2, and defining l3 to be distance between the x-y plane and the circular arc part 311 of a saddle-type encircling coil 3, distances l1, l2, and l3 are taken to be different values as shown in FIG. 2. The length of the circular arc part 111 (131) is the longest, the length of the circular arc part 311 (331) is intermediate, and the length of the circular arc part 211 (231) is the shortest. Each distance l1, l2 and l3 and each length of circular arc parts are determined by numerical calculations in order to optimize the gradient magnetic field linearity. By this way, an error of obtained gradient magnetic field linearity against ideal one is within at most several percent. In FIG. 2, as circular arc parts 131, 231 and 331 have a little influence on generating a gradient magnetic field, these parts are placed at the same position in the z axis. Each circular arc part of saddle-type encircling coil sets 1, 2 and 3 has almost the same size in its radius of curvature and all of these coils are mounted on a common frame.
As for a method for forming a gradient magnetic field coil by combining a plurality of sets of saddle-type encircling coils, these is another method using a larger number of saddle-type encircling coils as found in Japanese Patent application Laying-open No. 147450/1988 as well as aforementioned gradient magnetic field coil composed of three sets of saddle-type encircling coils which is disclosed in Japanese Patent Application Laying-open No. 229808/1987.
In aforementioned gradient magnetic field coils composed of a plurality of sets of saddle-type encircling coils, a set of saddle-type encircling coils composed of four saddle-type encircling coils placed symmetrical with respect to the x-y, y-z and z-x planes forms a primitive unit of coil equivalent to a one-turn coil. In case of forming a gradient magnetic field coil by combining a plurality of sets of this kind of saddle-type encircling coils, its ampere-turn is estimated to be a multiplication of the number of sets of saddle-type encircling coils to be used and an ampere-turn of each saddle-type encircling coil. An inductance of coils increases in proportion to the square of the number of turns of coils. As described above, a configuration of a gradient magnetic field coil by combining a plurality of sets of saddle-type encircling coils is aimed to generate a gradient magnetic field having a good linearity. In order to realize this configuration, it is required to use a set of saddle-type encircling coils, each coil having circular arc parts which are placed far from the x-y plane like a set 3 of saddle-type encircling coils as shown in FIG. 2. As these is a fact that the intensity of gradient magnetic field is getting larger as circular arc parts 111, 211 and 311 come closer to the x-y plane, the configuration of a set 3 of saddle-type encircling coils may not contribute to increasing the intensity of gradient magnetic field while this configuration mainly contributes to increasing the linearity in gradient magnetic field. So far, in the coil configuration as described above, there is a problem that the rate of increase of the strength of gradient magnetic field to the ampere-turn gets lower even if the number of turns of coils is increased in order to increase the linearity in gradient magnetic field.